The present invention relates generally to an exposure apparatus and method, and more particularly to an exposure apparatus equipped with an interferometer, and an exposure method using the exposure apparatus. The interferometer measures a wavefront aberration of a projection optical system for transferring a mask pattern onto an object to be exposed.
A projection exposure apparatus is conventionally used to transfer a mask or reticle pattern onto an object to be exposed in manufacturing devices, such as semiconductor devices, image pickup devices, display devices and magnetic heads, in the photolithography process. This exposure apparatus should transfer the mask pattern onto the object precisely at a predetermined magnification. For this purpose, it is important to use a projection optical system having excellent imaging performance and reduced aberration. In particular, due to the recent demands for finer processing to the semiconductor devices, a transferred pattern is more sensitive to the aberration of the optical system. Therefore, there is a need to measure the optical performance, such as a wavefront aberration, of the projection optical system with high precision. In addition, the simple, quick and inexpensive measurements are important for improved productivity and economical efficiency.
One known method for measuring the optical performance of the projection optical system actually exposes a mask pattern onto a wafer, and observes and inspects the resist image using a scanning electron microscope (“SEM”) or another means. This method, however, has a problem in a long inspection time due to the exposure and development, difficult SEM operations, and bad inspection reproducibility due to errors caused by resist applications and developments. Accordingly, as a solution for these problems, various measuring apparatuses have conventionally been proposed, such as a point diffraction interferometer (“PDI”) that has a pinhole used to form an ideal spherical wavefront, a shearing interferometer, such as a lateral shearing interferometer (“LSI”), or a Talbo interferometer that utilizes the shearing interferometry. More recently, a line diffraction interferometry (“LDI”) has been proposed which has a slit to form an ideal cylindrical wavefront or an ideal elliptical wavefront. See, for example, Japanese Patent Applications, Publication Nos. 57-64139, 2000-146705, and 2000-97666.
Nevertheless, the conventional entire system that includes the measuring apparatus and exposure apparatus separately is large and structurally complex, causing the increased cost and the long measuring time. Accordingly, applicant has already proposed an exposure apparatus equipped with an interferometer in Japanese Patent Application, Publication No. 2005-156506 (corresponding to U.S. patent application Ser. No. 11/167,112). In addition, the wavefront aberration of the projection optical system can be calculated by taking the phase information out of the interference fringe, for example, by using a fringe scan method. See for example, Optical Shop Testing second edition, Daniel Malacara, Wiley-Interscience Publication 1992, Chapter 14. “Phase Shifting Interferometry.”
In general, the highly coherent exposure light precludes accurate pattern transfer onto a wafer, because the lights that pass through the mask pattern interfere with each other. Therefore, the exposure apparatus usually makes the exposure light incoherent using the illumination optical system. However, in an exposure apparatus equipped with an interferometer, the interferometer should utilize the low coherent exposure light, thereby deteriorating the precision of the wavefront aberration. Accordingly, the instant inventors have studied an application of the light from an alignment scope for an alignment between a mask and a wafer. However, the inventors have discovered that the light from the alignment scope is also made incoherent to some extent so as to reduce speckles, and poses a similar problem. In addition, the fringe scan method needs to change a phase of the light and thus a driving system for this purpose provided in the exposure apparatus causes a complicated structure and increased cost. There is a need to measure the wavefront aberration using a simple structure. In some instances, there is a demand to eliminate an uneven light intensity distribution and to improve the measuring precision of the wavefront aberration even at cost of the coherence.